1. Field of the Invention
The present invention relates to technology for amplifying light using a nonlinear optical effect, and in particular relates to an optical parametric amplifier which amplifies signal light using an optical parametric effect which is produced by four wave mixing or the like.
2. Background Art
In previous and present optical communication systems, a single wavelength transmission system of 10 Gb/s, or a wavelength-division multiplexed (WDM) transmission system have been developed which compensate for a decay in the signal light power due to losses of the transmission path fiber, by means of an optical amplifier such as an optical fiber amplifier.
One large problem at present is that there is a limit to the amplification band of the actual optical fiber amplifier. More specifically, in an erbium (Er3+) doped fiber amplifier (EDFA), the amplifying range in the C-band in the vicinity of 1535 nm to 1565 nm, and in the L-band in the vicinity of 1570 nm to 1610 nm is limited. In order to amplifying bands other than the C- and L-bands, there has been proposed an optical fiber amplifier or the like which is doped with a material other than Er3+, however the present situation is that this has not yet been obtained the feasibility of an EDFA.
Furthermore, in the EDFA, the noise figure is limited to 5 to 6 dB or thereabove, so that there is also the problem that the influence of the reduction in optical S/N ratio due to the added amplified spontaneous emission (ASE) is severe. This becomes a large limiting factor in the long distance transmission of signal light which is to be sped up in the future to 40 Gb/s or 160 Gb/s by limiting the repeater interval in the optical transmission system which uses an optical amplifier. The actual transmission distance in present systems is limited to several 100 kilometers for signal light of 40 Gb/s, and to several kilometers at most for signal light of 160 Gb/s.
To address the above problem of optical fiber amplifiers such as EDFAs, as an optical amplifying technique aimed at covering a wider band, for example an optical Raman amplifier has been proposed which uses a stimulated Raman scattering, being one of the nonlinear optical effects. Furthermore, there is also known a technique for amplifying signal light using an optical parametric amplification effect which is produced by four wave mixing (FWM) or the like, and various types of optical devices which adopt this optical parametric amplification have been proposed (for example refer to PCT International Publication No. WO 94/09403 pamphlet, Japanese Unexamined Patent Publication Nos. 7-98464, and 2006-184851, and Watanabe et al., “Novel Fiber Kerr-Switch with Parametric Gain: Demonstration of Optical Demultiplexing and Sampling up to 640 Gb/s”, ECOC. 2004, Th 4.1.6.).
However, the above conventional optical Raman amplifier is a gain-distributed type optical amplifier which uses a part or all of the transmission path fiber as an amplification medium, so that there is the problem that the maintenance and administration of the transmission path is extremely difficult.
On the other hand, in an optical amplifier which uses the optical parametric amplification effect, the construction is basically different to that of the gain-distributed type optical amplifier. Therefore the abovementioned problems with the maintenance and administration do not arise. Furthermore, there is less deterioration in the optical S/N ratio due to ASE noise compared with the EDFA. Therefore it is possible to realize favorable noise characteristics. However, the apparatus proposed up until now which uses the optical parametric amplification effect is limited to a configuration in which it is presumed that the wavelength of signal light corresponding to the amplification band of the actual optical amplifier such as the EDFA or the like, that is to say the signal light of the C-band or the L-band or the like is amplified, and there has yet to be a proposal for a specific and practical configuration for amplifying the signal light outside of the amplification band of the conventional optical amplifier.